This invention relates to optical coherence tomography (OCT), specifically to fiber OCT with coherence scanning in three dimensions.
OCT known in the prior art typically involves one-dimensional coherence gating (scanning) in what will be illustrated herein as the “z” dimension, combined with mechanical scanning in the two remaining “x” and “y” dimensions, to produce a 3D image of a scattering object The invention to be disclosed herein provides an OCT apparatus with coherence scanning in all three dimensions and a method for implementation of such a scanning.
The literature contains numerous papers regarding various types of OCT and related scanning. For example:
S. A. Boppart, W. Drexler, U. Morgner, F. Kartner, J. Fujimoto, Ultrahigh Resolution and Spectroscopic OCT Imaging of Cellular Morphology and Function, Proc. Inter-Institute Workshop on In Vivo Optical Imaging at the National Institutes Health. Ed. A. H. Gandjbakhche. September 16-17, pp. 56-61, 1999, describes OCT with the femtosecond Cr:Forsterite laser tunable over wavelengths from 1230 to 1270 nm, providing structurally resolved spectroscopic information.
S. A. Boppart, W. Drexler, U. Morgner, F. X. Kartner, J. G. Fujimoto, Ultrahigh Resolution and Spectroscopic Optical Coherence Tomography Imaging of Cellular Morphology and Function, Proc. Inter-Institute Workshop on In Vivo Optical Imaging at the National Institutes of Health. Ed. Gandjbakhche A H. September 16-17, pp. 56-61, 1999, describes endoscopic in vitro OCT of specimens of Barrett's esophagus.
J. G. Fujimoto, S. A. Boppart, C. Pitris, M. E. Brezinski, Optical coherence tomography a new technology for biomedical imaging, Japanese Journal of Laser Surgery and Medicine 20:141-168, 199; and J. G. Fujimoto, M. E. Brezinski, G. J. Tearney, S. A. Boppart, B. E. Bouma, M. R. Hee, J. F. Southern, E. A. Swanson, Biomedical imaging and optical biopsy using optical coherence tomography, Nature Med 1(9):970-972, 1995, describe OCT to non-excisionally evaluate tissue morphology using a catheter or an endoscope.
G. J. Tearney, S. A. Boppart, B. E. Bouma, M. E. Brezinski, N. J. Weissman, J. F. Southern, J. G. Fujimoto, Scanning single-mode fiber optic catheter-endoscope for optical coherence tomography, Opt Lett 21(7):543-545, 1996, describes an OCT catheter-endoscope for micrometer-scale, cross-sectional imaging in internal organ systems. A device with a diameter as small as 1.1 mm has been achieved, and imaging of in vitro human venous morphology is demonstrated. Like much of the prior art, OCT is supplemented with mechanical scanning.
M. Lazebnik, D. L. Marks, K. Potgieter, R. Gillette, S. A. Boppart, Functional optical coherence tomography for detecting neural activity through scattering changes, Optics Letters, 28(14):1218-1220, 2003, describes functional optical coherence tomography (fOCT) for neural imaging by detecting scattering changes during the propagation of action potentials through neural tissue.
D. L. Marks, A. L. Oldenburg, J. J. Reynolds, S. A. Boppart, Autofocus algorithm for dispersion correction in optical coherence tomography, Applied Optics 42:3038-3046, 2003, describes an autofocus algorithm for estimating the delay line and material dispersion from OCT reflectance data based on minimizing the Renyi entropy of the corrected axial-scan image, which is a contrast-enhancement criterion. This autofocus algorithm can be used in conjunction with a high-speed, digital-signal-processor-based OCT acquisition system for rapid image correction.
A. L. Oldenburg, J. J. Reynolds, D. L. Marks, S. A. Boppart, Fast-Fourier-domain delay line for in vivo optical coherence tomography with a polygonal scanner, Apl. Opt., 42(22):4606, August, 2003, describes in vivo optical coherence tomography using a Fourier-domain optical delay line constructed with a commercially available polygonal scanner.
Zysk. A., J. J. Reynolds, D. L. Marks, P. S. Carney, S. A. Boppart, Projected index computed tomography, Opt. Letters 28:701-703, 2003, describes optical coherence tomography with images taken from several view angles to determine a mapping of the refractive indices of the sample.
Li, X., Boppart, S. A, J. Van Dam, H. Mashimo, M. W. Mutinga, W. Drexler, M. Klein, C. Pitris, M. L. Krinsky, M. E. Brezinski, J. G. Fujimoto, Optical coherence tomography: advanced technology for the endoscopic imaging of Barrett's esophagus, Endoscopy 32:921-930, 2000, describes OCT technology with compact fiberoptic imaging probes that can be used as an adjunct to endoscopy for real-time image-guided evaluation of Barrett's esophagus. Linear and radial (mechanical) scan patterns have different advantages and limitations depending upon the application.
J. A. Izatt, M. D. Kulkarni, S. Yazdanfar, J. K. Barton, A. J. Welch, In vivo bidirectional Doppler flow imaging of picoliter blood volumes using optical coherence tomography, Optics Letters, 22(18), 1439-1441, 1997, describes OCT with flow detection based on Doppler effect.
J. K. Barton, J. A. Izatt, A. J. Welch, Investigating pulsed dye laser-blood vessel interaction with color Doppler optical coherence tomography, Optics Express 2, 251-256, 1998, describes OCT with flow detection based on Doppler effect
J. A. Izatt, M. D. Kulkarni, K. Kobayashi, M. V. Sivak, J. K. Barton, and A. J. Welch, Optical Coherence Tomography for Biodiagnostics, Optics and Photonics News 8:41-47, 1997, describes application of OCT in diagnostics
M. Laubscher, M. Ducros, B. Karamata, T. Lasser, R. Salathe, Video-rate three-dimensional optical coherence tomography. Optics Express, Vol. 10 Issue 9 Page 429, May 2002, describes three-dimensional optical coherence tomography (3D OCT) at video rate. A 58 by 58 smart-pixel detector array was employed. A sample volume of 210×210×80 m3 (corresponding to 58×58×58 voxels) was imaged at 25 Hz.
L. Vabre, A. Dubois, A. C. Boccara, Thermal-light full-field optical coherence tomography, Optics Letters, Vol. 27 Issue 7 Page 530, April 2002, describes OCT system based on a Linnik-type interference microscope, illuminated by a white-light thermal lamp.
A. Dubois, L. Vabre, A. C. Boccara, E. Beaurepaire, High-Resolution Full-Field Optical Coherence Tomography with a Linnik Microscope, Applied Optics, Vol. 41 Issue 4 Page 805, February 2002, describes an OCT system based on a Linnik interference microscope with high-numerical-aperture objectives. Lock-in detection of the interference signal is achieved in parallel on a CCD by use of a photoelastic birefringence modulator and full-field stroboscopic illumination with an infrared LED.
V. X. Yang, M. L. Gordon, B. Qi, J. Pekar, S. Lo, E. Seng-Yue, A. Mok, B. C. Wilson, and I. A. Vitkin, High speed, wide velocity dynamic range Doppler optical coherence tomography (Part I): System design, signal processing, and performance, Opt. Express 11, 794-809, 2003, describes improvements in real-time Doppler optical coherence tomography (DOCT), acquiring up to 32 frames per second at 250×512 pixels per image.
V. X. Yang, M. L. Gordon, E. Seng-Yue, S. Lo, B. Qi, J. Pekar, A. Mok, B. C. Wilson, and I. A. Vitkin, High speed, wide velocity dynamic range Doppler optical coherence tomography (Part II): Imaging in vivo cardiac dynamics of Xenopus laevis, Opt. Express 11, 1650-1658, 2003, describes DOCT that can detect changes in velocity distribution during heart cycles, measure the velocity gradient in the embryo, and distinguish blood flow Doppler signal from heart wall motions.
D. Huang, E. A. Swanson, C. P. Lin, J. S. Schuman, W. G. Stinson, W. Chang, M. R. Hee, T. Flotte, K. Gregory, C. A. Puliafito, and J. G. Fujimoto, Optical coherence tomography, Science 254, 1178-81, 1991, describes the principles of OCT.
F. I. Feldchtein, G. V. Gelikonov, V. M. Gelikonov, R. V. Kuranov, A. M. Sergeev, N. Gladkova, A. V. Shakhov, N. M. Shakhova, L. B. Snopova, A. B. Terent'eva, E. V. Zagainova, Y. P. Chumakov, and I. A. Kuznetzova, Endoscopic applications of optical coherence tomography, Opt. Express 3, 257-70 1998, describes application of an endoscopic OCT system in clinical experiments to image human internal organs.
C. Vinegoni, J. S. Bredfeldt, D. L. Marks, S. A. Boppart., Nonlinear optical contrast enhancement for optical coherence tomography, Optics Express, Vol. 12 Issue 2 Page 331, January 2004, describes interferometric technique for measuring Coherent Anti-Stokes Raman Scattering (CARS) and Second Harmonic Generation (SHG) signals. Heterodyne detection is employed to increase the sensitivity in both CARS and SHG signal detection.
P. Yu, L. Peng, M. Mustata, J. J. Turek, M. R. Melloch, D. D. Nolte, Time-dependent speckle in holographic optical coherence imaging and the health of tumor tissue, Optics Letters, Vol. 29 Issue 1 Page 68, January 2004, describes holographic optical coherence imaging acquiring en face images from successive depths inside scattering tissue.
S. H. Yun, G. J. Tearney, B. E. Bouma, B. H. Park, J. F. de Boer, High-speed spectral-domain optical coherence tomography at 1.3 Î¼m wavelength, Optics Express, Vol. 11 Issue 26 Page 3598, December 2003, describes a high-speed spectral domain optical coherence tomography (SD-OCT) system capable of acquiring individual axial scans in 24.4 μs at a rate of 19,000 axial scans per second.
M. Laubscher, L. Froehly, B. Karamata, R. P. Salath, T. Lasser, Self-referenced method for optical path difference calibration in low-coherence interferometry, Optics Letters, Vol. 28 Issue 24 Page 2476, December 2003, describes a method for the calibration of optical path difference modulation in low-coherence interferometry. Spectrally filtering a part of the detected interference signal results in a high-coherence signal that encodes the scan imperfections and permits their correction.
A. V. Zvyagin, K. K. M. B. Dilusha Silva, S. A. Alexandrov, T. R. Hillman, J. J. Armstrong, T. Tsuzuki, D. D. Sampson, Refractive index tomography of turbid media by bifocal optical coherence refractometry, Optics Express, Vol. 11 Issue 25 Page 3503, December 2003, describes tomographic imaging of the refractive index of turbid media using bifocal optical coherence refractometry (BOCR). The technique, which is a variant of optical coherence tomography, is based on the measurement of the optical pathlength difference between two foci simultaneously present in a medium of interest.
M. A. Choma, C. Yang, J. A. Izatt, Instantaneous quadrature low-coherence interferometry with 33 fiber-optic couplers, Optics Letters, Vol. 28 Issue 22 Page 2162, November 2003, describes fiber-based quadrature low-coherence interferometers that exploit the inherent phase shifts of 33 and higher-order fiber-optic couplers.
R. A. Leitgeb, C. K. Hitzenberger, A. F. Fercher, T. Bajraszewski, Phase-shifting algorithm to achieve high-speed long-depth-range probing by frequency-domain optical coherence tomography, Optics Letters, Vol. 28 Issue 22 Page 2201, November 2003, describes how by exploiting the phase information of the recorded interferograms, it is possible to remove autocorrelation terms and to double the measurement range in OCT.
S. H. Yun, G. J. Tearney, J. F. de Boer, N. Iftimia, B. E. Bouma, High-speed optical frequency-domain imaging, Optics Express, Vol. 11 Issue 22 Page 2953, November 2003, describes high-speed, high-sensitivity, high-resolution optical imaging based on optical frequency-domain interferometry using a rapidly-tuned wavelength-swept laser.
C. K. Hitzenberger, P. Trost, P. W. Lo, Q. Zhou, Three-dimensional imaging of the human retina by high-speed optical coherence tomography, Optics Express, Vol. 11 Issue 21 Page 2753, October 2003, describes a technique that combines the transverse scanning approach of a confocal scanning laser ophthalmoscope with the depth sectioning capability of OCT. A stable high-frequency carrier is generated by use of an acousto optic modulator, and high frame rate is obtained by using a resonant (mechanical) scanning mirror for the priority scan (x-direction).
V. X. D. Yang, M. L. Gordon, S. J. Tang, N. E. Marcon, G. Gardiner, B. Qi, S. Bisland, E. Seng-Yue, S. Lo, J. Pekar, B. C. Wilson, I. A. Vitkin, High speed, wide velocity dynamic range Doppler optical coherence tomography, Optics Express, Vol. 11 Issue 19 Page 2416, September 2003, describes inhibition of the sidelobes of the axial point spread function in optical coherence tomography by shaping the power spectrum of the light source with a remaining power of 4.54 mW. A broadband amplified spontaneous emission source radiating at 156540 nm is employed in a free-space optical coherence tomography system.
N. A. Riza, Z. Yaqoob, Submicrosecond Speed Optical Coherence Tomography System Design and Analysis by use of Acousto-Optics, Applied Optics, Vol. 42 Issue 16 Page 3018, June 2003, describes a high-speed no-moving-parts optical coherence tomography (OCT) system that acquires sample data at less than a microsecond per data point sampling rate. The basic principle of the proposed OCT system relies on use of an acousto-optic deflector.
Y. Wang, J. S. Nelson, Z. Chen, B. J. Reiser, R. S. Chuck, R S. Windeler, Optimal wavelength for ultrahigh-resolution optical coherence tomography, Optics Express, Vol. 11 Issue 12 Page 1411, June 2003, describes the influence of depth dependent dispersion by the main component of biological tissues, water, on the resolution of OCT.
M. Akiba, K. P. Chan, N. Tanno, Full-field optical coherence tomography by two-dimensional heterodyne detection with a pair of CCD cameras, Optics Letters, Vol. 28 Issue 10 Page 816, May 2003, describes a two-dimensional heterodyne detection technique based on the frequency-synchronous detection method [Jpn. J. Appl. Phys. 39, 1194, 2000] which is demonstrated for full-field optical coherence tomography. This technique, which employs a pair of CCD cameras to detect the in-phase and quadrature components of the heterodyne signal simultaneously, offers the advantage of phase-drift suppression in interferometric measurement.
R. A. Leitgeb, C. K. Hitzenberger, A. F. Fercher, Performance of Fourier domain vs. time domain optical coherence tomography. Optics Express, Vol. 11 Issue 8 Page 889, April 2003, presents a discussion of noise sources in Fourier Domain Optical Coherence Tomography (FDOCT) setups. The performance of FDOCT with charge coupled device (CCD) cameras is compared to current standard time domain OCT systems.
B. Povazay, K. Bizheva, A. Unterhuber, B. Hermann, H. Sattmann, A. F. Fercher, W. Drexler, A. Apolonski, W. J. Wadsworth, J. C. Knight, P. St. J. Russell, M. Vetterlein, E. Scherzer. Submicrometer axial resolution optical coherence tomography, Optics Letters, Vol. 27 Issue 20 Page 1800, October 2002, describes OCT with submicrometer axial resolution achieved by use of a photonic crystal fiber in combination with a sub-10-fs Tisapphire laser.
Y. Lu, H. Lei, Q. Pan, Z. Liu, G. L. Rempel, Holographic coherence tomography for measurement of three-dimensional refractive-index space, Optics Letters, Vol. 27 Issue 13 Page 1102, July 2002, describes a tomographic method for the measurement of three-dimensional refractive-index fields of transparent media, called holographic coherence tomography, which combines double-exposure holographic interferometry with the detecting style of optical coherence tomography. The three-dimensional refractive-index field can be achieved with a confocal lens system by continual longitudinal and horizontal scanning of the holographic reconstruction image.
N. G. Chen, Q. Zhu, Rotary mirror array for high-speed optical coherence tomography, Optics Letters, Vol. 27 Issue 8 Page 607, April 2002, describes a high-speed, high-duty-cycle, linear optical delay line suitable for optical coherence tomography and optical Doppler tomography. Periodic longitudinal scanning is achieved by use of a tilted mirror array rotating at a constant speed.
A. V. Zvyagin, I. Eix, D. D. Sampson, High-Speed High-Sensitivity, Gated Surface Profiling with Closed-Loop Optical Coherence Topography, Applied Optics, Vol. 41 Issue 11 Page 2179, April 2002, describes a surface profiling technique with a closed-loop optical coherence topography. This technique is a scanning beam, servo-locked variation of low-coherence interferometry.
Y. Yasuno, Y. Sutoh, M. Nakama, S. Makita, M. Itoh, T. Yatagai, M. Mori, Spectral interferometric optical coherence tomography with nonlinear b-barium borate time gating, Optics Letters, Vol. 27 Issue 6 Page 403, March 2002, describes a high-speed, all optical coherence tomography system. This tomography system employs spectral interferometry and optical Fourier transformation to reduce the number of mechanical scanning dimensions required for multidimensional profilometry.
C. K. Hitzenberger, M. Sticker, R. Leitgeb, A. F. Fercher, Differential phase measurements in low-coherence interferometry without 2 p ambiguity, Optics Letters, Vol. 26 Issue 23 Page 1864, December 2001, describes a method that overcomes the ambiguity of quantitative phase measurements by low-coherence interferometry and optical coherence tomography due to restriction by the well-known 2 p ambiguity to path-length differences smaller than ½.
S. Bourquin, P. Seitz, R. P. Salath, Optical coherence topography based on a two-dimensional smart detector array, Optics Letters, Vol. 26 Issue 8 Page 512, April 2001, describes a low-coherence reflectometer based on a conventional Michelson interferometer and a novel silicon detector chip with a two-dimensional array of pixels that allows parallel heterodyne detection. Acquisition of three-dimensional images with more than 100,000 voxels per scan at a sensitivity of −58 dB and a rate of 6Hz is demonstrated.
Y. Zhang, M. Sato, N. Tanno, Resolution improvement in optical coherence tomography by optimal synthesis of light-emitting diodes, Optics Letters, Vol. 26 Issue 4 Page 205, February 2001, describes an approach to improving the longitudinal resolution of optical coherence tomography in free space, by optimal synthesis of several LED's.
Y. Yasuno, M. Nakama, Y. Sutoh, M. Itoh, T. Yatagai, M. Mori, Phase-resolved correlation and its application to analysis of low-coherence interferograms, Optics Letters, Vol. 26 Issue 2 Page 90, January 2001, describes a signal-processing technique that involves a phase-resolved correlation method to determine the phase distribution of low-coherence interferograms. This method improves the sensitivity and resolution of low-coherence interferometers.
A. V. Zvyagin, J. B. Fitzgerald, K. K. M. B. D. Silva, D. D. Sampson, Real-time detection technique for Doppler optical coherence tomography, Optics Letters, Vol. 25 Issue 22 Page 1645, November 2000, describes a detection technique, based on a modified electronic phase-locked loop, for Doppler optical coherence tomography. The technique permits real-time simultaneous reflectivity and continuous, bidirectional velocity mapping in turbid media over a wide velocity range with minimal sensitivity penalty compared with conventional optical coherence tomography.
S. Bourquin, V. Monterosso, P. Seitz, R. P. Salath. Video-rate optical low-coherence reflectometry based on a linear smart detector array, Optics Letters, Vol. 25 Issue 2 Page 102, January 2000, describes a low-coherence reflectometer based on a conventional Michelson interferometer and a novel silicon detector chip that allows parallel heterodyne detection. Cross-sectional images of 64256 pixels covering an area of 1.92 mm 1.3 mm are acquired at video rate and with a sensitivity close to the shot-noise limit.
L. Giniunas, R Danielius, R Karkockas. Scanning Delay Line with a Rotating-Parallelogram Prism for Low-Coherence Interferometry, Applied Optics, Vol. 38 Issue 34 Page 7076, December 1999, describes a fast scanning optical delay line for low-coherence interferometry that has good linearity, a high duty cycle, and a continuously adjustable scan range. The delay line consists of a rotating-parallelogram prism with the rotation axis tilted with respect to the incident beam and two motionless mirrors.
A. M. Rollins, J. A. Izatt, Optimal interferometer designs for optical coherence tomography, Optics Letters, Vol. 24 Issue 21 Page 1484, November 1999, describes a family of power-conserving fiber-optic interferometer designs for low-coherence reflectometry that use optical circulators, unbalanced couplers, and/or balanced heterodyne detection.
C. K. Hitzenberger, A. F. Fercher, Differential phase contrast in optical coherence tomography, Optics Letters, Vol.24 Issue 9 Page 622, May 1999, describes a modification of optical coherence tomography (OCT) that allows one to measure small phase differences between beams traversing adjacent areas of a specimen. The sample beam of a polarization-sensitive low-coherence interferometer is split by a Wollaston prism into two components that traverse the object along closely spaced paths.
Despite the wealth of literature in this area, none of these papers describes 3D coherence scanning. Most of them use mechanical scanning, one (Nabeel Riza) uses acousto-optical scanning (a straight replacement of a mechanical scanner by an AO one), or CCD-type 2D sensors which are not applicable for small, fiberoptic probes.
There are also a air number of U.S. patents dealing various aspect of OCT and related scanning, yet here too, 3D coherence scanning appears to be novel and inventive in relation to all of the prior art.
U.S. Pat. No. 6,325,512 discloses an OCT apparatus with eye tracking. U.S. Pat. No. 6,507,747 discloses an optical imaging probe for providing information representative of morphological arid biochemical properties of a sample. U.S. Pat. No. 6,556,854 discloses a blood vessel imaging system using homodyne and heterodyne effects. U.S. Pat. No. 6,654,630 discloses a catheter based optical system for generating data as to the condition of a tissue sample of a mammalian vessel. U.S. Pat. No. 5,257,991 discloses a method and apparatus for directing light at an angle and provides an optical fiber with a beveled end. U.S. Pat. No. 5,343,543 discloses a side firing laser fiber. U.S. Pat. No. 5,354,294 discloses a fiber optic laser beam angle delivery. U.S. Pat. No. 5,361,130 discloses a fiber grating-based sensing system with interferometric wavelength-shift detection. U.S. Pat. No. 5,366,456 discloses an angle firing fiber optic laser. U.S. Pat. No. 5,370,649 discloses a laterally reflecting tip for laser transmitting fiber. U.S. Pat. No. 5,401,270 discloses an applicator device for laser radiation. U.S. Pat. No. 5,402,236 discloses a fiberoptic displacement sensor using interferometric techniques. U.S. Pat. No. 5,428,699 discloses a probe having optical fiber for laterally directing a laser beam. U.S. Pat. No. 5,439,000 discloses an apparatus for diagnosing tissue with a guidewire. U.S. Pat. No. 5,490,521 discloses an ultrasound biopsy needle. U.S. Pat. No. 5,495,541 discloses an optical delivery device with high numerical aperture curved waveguide. U.S. Pat. No. 5,509,917 discloses lensed caps for radial medical laser delivery devices. U.S. Pat. No. 5,537,499 discloses a side-firing laser optical fiber probe. U.S. Pat. No. 5,562,657 discloses a side firing laser catheter. U.S. Pat. No. 5,571,099 discloses a side firing optical probe. U.S. Pat. No. 5,601,087 discloses a guidewire and a fiber probe. U.S. Pat. No. 5,633,712 discloses a reflectometer for determining the thickness and index of refraction of a film. U.S. Pat. No. 5,772,657 discloses a side firing fiber optic laser probe. U.S. Pat. No. 6,091,496 discloses an apparatus for confocal interference microscopy using wavenumber domain reflectometry. U.S. Pat. No. 6,160,826 discloses a method and apparatus for performing optical frequency domain reflectometry using a tunable laser. U.S. Pat. No. 6,243,169 discloses an interferometric instrument for sensing the surfaces of a test object. U.S. Pat. No. 6,252,669 discloses an interferometric instrument provided with an arrangement for periodically changing a light path of a received beam component. U.S. Pat. No. 6,381,023 discloses an improved confocal microscope system which images sections of tissue utilizing heterodyne detection. U.S. Pat. No. 6,423,956 discloses an angled-dual-axis confocal scanning microscope comprising a fiber-coupled, angled-dual-axis confocal scanning head and a vertical scanning unit. U.S. Pat. No. 6,441,356 discloses an angled-dual-axis optical coherence scanning microscope comprising a fiber-coupled, high-speed angled-dual-axis confocal scanning head and a vertical scanning unit. U.S. Pat. No. 6,445,939 discloses ultra-small optical probes comprising a single-mode optical fiber and a lens which has substantially the same diameter as the optical fiber. U.S. Pat. No. 6,466,713 discloses the head of an optical fiber comprising the sensing probe of an optical heterodyne sensing device.
U.S. Pat. Nos. 6,525,823; 6,559,950; 6,587,206; and 6,590,664 all disclose an optical system for monitoring a colloidal suspension.
U.S. Pat. No. 5,094,534 discloses an optical layout similar to OCT, with the main purpose of measurement of displacement of a sensor diaphragm. However, no 3D image appears to be produced by the apparatus. U.S. Pat. No. 5,202,745 discloses a reflectometer with the layout similar to OCT. In the invention, modulation of phase by PZT appears serving to produce signal spectral component of the same frequency as a fixed filter frequency in a detector, for any given mirror velocity. No 3D image is formed.
U.S. Pat. No. 5,268,738 discloses a reflectometer with multiple reference planes for increased depth range. U.S. Pat. No. 5,268,741 discloses a reflectometer, bearing similarity to OCT. Modulation of the source is introduced for calibration. U.S. Pat. No. 5,291,267 discloses a reflectometer, with similarity to OCT, comprising an amplifier for better sensitivity and reduced measurement times. U.S. Pat. No. 5,365,335 discloses a reflectometer with similarity to OCT in the optical layout. U.S. Pat. No. 5,459,570 discloses an optical coherence domain reflectometer employing two or more wavelengths of light. Polarization sensitivity provides for measurement of birefringence. The apparatus employs modulation of light and a fixed filter. U.S. Pat. No. 5,719,673 discloses an apparatus for OCT characterization of a sample in a single spot. The specific application is for measuring properties of different layers in the eye.
U.S. Pat. No. 5,731,876 discloses a single spot OCT for measurement properties of layered films. U.S. Pat. No. 5,835,642 discloses an optical delay line with all-fiber design. U.S. Pat. No. 5,867,268 also discloses an optical delay line with all-fiber design. U.S. Pat. No. 6,111,645 discloses an optical delay line. U.S. Pat. No. 6,381,490 discloses an optical scanning and imaging system and related method for scanning and imaging an object. U.S. Pat. No. 6,564,087 discloses a fiberoptic needle probe, which involves mechanical rotation. U.S. Pat. No. 5,383,467 discloses a guidewire catheter and apparatus for diagnostic imaging. The apparatus is applicable to OCT and has a small probe diameter. U.S. Pat. No. 5,570,182 discloses an OCT apparatus with acousto-optic or PZT modulation for heterodyning. It includes a movable lens for mechanical scan in the two dimensions normal to the optical axis. U.S. Pat. No. 5,579,112 discloses a free space, non-fiberoptic, OCT apparatus. U.S. Pat. No. 5,847,827 discloses a conventional OCT apparatus where the axial (z) coordinate of the focal spot follows the coherence scan. U.S. Pat. No. 5,920,390 discloses an OCT apparatus with wavelength selectivity to characterize tissue based on the reflection spectrum. U.S. Pat. No. 5,921,926 discloses an OCT apparatus with simultaneous spectral interferometry capabilities. The design involves mechanical rotation of the fiber/lenslet array. U.S. Pat. No. 5,975,697 discloses an OCT apparatus with adjustable coherence properties and adjustable depth resolution. U.S. Pat. No. 6,006,128 discloses an OCT apparatus with Doppler flow imaging. U.S. Pat. No. 6,053,613 discloses a modified OCT interferometer. U.S. Pat. No. 6,057,920 discloses an OCT apparatus with focal spot z-position following the coherence scan. U.S. Pat. No. 6,069,698 discloses an OCT apparatus with light path adjusted by the uniaxial stage such that the beam interference is detected for the scan range, to ensure stable acquisition of tomographic images. The design involves mechanical motion of components.
U.S. Pat. No. 6,124,930 discloses an improvement on conventional OCT. The invention stabilizes frequency of the signal with a transverse scan. U.S. Pat. No. 6,137,585 discloses an apparatus for generating data representative of a three-dimensional distribution of the light backscattering potential of a transparent or semi-transparent object such as a human eye. The signal processing involves frequency shift by acousto-optical elements and heterodyning. One spot in the sample produces reference, another (moving) spot produces signal. This design decreases sensitivity to eye movement. U.S. Pat. No. 6,141,577 discloses an OCT apparatus with simultaneous spectral interferometry capabilities. The design involves mechanical rotation of the fiber/lenslet array. U.S. Pat. No. 6,175,669 discloses an optical coherence domain reflectometry guidewire with multiplexed fiber channels and both forward and side viewing. U.S. Pat. No. 6,191,862 discloses a conventional OCT apparatus with improved high-speed axial (z) scanning. U.S. Pat. No. 6,198,540 discloses an improved OCT apparatus with a plurality of reference planes, frequency-multiplexed, and high acquisition speed.
U.S. Pat. Nos. 6,201,608; 6,233,055; 6,252,666; and 6,307,633 all disclose an improved conventional OCT using a polarizing beam splitter and a polarization rotator to improve signal to noise ratio.
U.S. Pat. No. 6,384,915 discloses multiplexed optical coherence reflectometry with multiple fibers for guidance and viewing. U.S. Pat. No. 6,421,164 discloses a high speed conventional OCT apparatus with mechanical scanning. U.S. Pat. No. 6,485,413 discloses an imaging system for performing forward scanning imaging for application to therapeutic and diagnostic devises used in medical procedures. U.S. Pat. No. 6,501,551 discloses an imaging system for performing optical coherence tomography. The endoscopic device involves mechanical rotation and longitudinal scanning. U.S. Pat. No. 6,552,796 discloses a conventional OCT apparatus for reading information from a desired depth in a sample. U.S. Pat. No. 6,552,797 discloses an apparatus for measurement of the freezing point of substances. U.S. Pat. No. 6,564,089 discloses a conventional OCT apparatus with a Faraday rotator for stabilization of polarization and image contrast. U.S. Pat. No. 6,608,684 discloses an OCT apparatus with magnetic mechanical lateral scanning of a fiber. U.S. Pat. No. 6,608,717 discloses rapid in-vivo optical coherence microscopy with a conventional OCT layout. U.S. Pat. No. 6,615,072 discloses a conventional OCT apparatus with a Faraday rotator for stabilization of polarization and image contrast. U.S. Pat. No. 6,618,152 discloses an optical coherence tomography apparatus using optical-waveguide structure which reduces pulse width of low-coherence light frequency shift between reference and sample.
U S. Pat. No. 6,628,401 discloses an optical tomography imaging method and apparatus with amplified spontaneous emission light. This is yet another implementation of conventional OCT with mechanical scanning. U.S. Pat. No. 6,657,727 discloses interferometers for optical coherence domain reflectometry and optical coherence tomography using nonreciprocal optical elements. The design uses differential signal processing. It involves mechanical scanning, as in conventional OCT. U.S. Pat. No. 6,611,338 discloses an OCT apparatus using light of two wavelengths and amplitude modulation. The design involves mechanical scanning. U.S. Pat. No. 6,636,755 discloses a conventional OCT apparatus with high resolution for cellular imaging. The design requires mechanical scanning. U.S. Pat. No. 5,321,501 discloses a classic OCT apparatus by Fujimoto, et al. The apparatus requires 2D mechanical motion of either sample or the probe (part of the probe). U.S. Pat. No. 6,134,003 discloses an endoscopic OCT device, including a variety of probes. All embodiments involve mechanical motion.
U.S. Pat. No. 5,465,147 discloses an OCT apparatus. The 3D image of the sample is produced by a coherence scan in one (z) dimension and 2D (x,y) conventional imaging with a CCD. A drawback of the apparatus is the potentially low-frequency detection typical of CCD.
U.S. Pat. No. 5,994,690 discloses an OCT apparatus with deconvolution for improved axial resolution. The extent of resolution improvement was limited due to noise, as described in M. D. Kulkarni, C. W. Thomas, and J. A. Izatt, Image enhancement in optical coherence tomography using deconvolution, Electron. Lett., vol. 33, pp. 1365-1367, 1997.
U.S. Pat. No. 6002,480 discloses OCT with deconvolution, similar to U.S. Pat. No. 5,994,690.
U.S. Pat. No. 6,295,132 discloses a reflectometer with acousto-optical deflectors and gratings for transverse displacement of the beam. The device is inherently large and is not fiberoptic.
U.S. Pat. No. 5,555,087 discloses a 3D OCT apparatus employing a bundle of fibers, each producing the conventional OCT. The bundle replaces the mechanical 2D scan of other prior art such as in patents to Fujimoto and others. The drawbacks include large probe size, potentially slower operating with the CCD-type detector, and likely cross-talk between fibers producing artifacts in the output image.
We now proceed to a review of optical coherence tomography.
The basic layout of optical coherence tomography is shown in FIG. 1. A Light Source 110 has a low coherence length Lc, related to the spectral width
            Δ      ⁢                          ⁢      λ      ⁢              :            ⁢              L        c              ≈                  λ        2                    Δ        ⁢                                  ⁢        λ              ,where λ is central wavelength. Lc is typically ˜10 μm when super-luminescent diode serves as the light source. Typical instrumental function of the OCT is presented in FIG. 2.
The instrumental function (impulse response) is defined as the signal produced by scanning (displacement) of a reflector 116 on a photo detector 124 of FIG. 1 when the object 122 is a small point. The z-displacement 20 of the reflector 116 is scanned from the position when the optical path length from the light source 110 is exactly the same to the object 122 and to the reflector 116. The scanning range is typically comparable with the coherence length of the light source. An electromechanical device responsive to electromagnetic signals 118 such as but not limited to a piezoelectric stack is one possible means for causing reflector displacement 126, thereby introducing a phase delay. As the reflector is displaced, interference between the light scattered back from a point of object 122 and the light reflected by the reflector 116 produces z-fringes 142, As the displacement increases, the contrast of z-fringes 142 reduces due to loss of coherence (coherence gating). The width of the z-envelope 144 is defined by the coherence length Lc. Instrumental function, I (21) defines the resolution in z (depth) as the depth is scanned by displacement of the reflector 116. Lateral scanning of the sample is done mechanically, by displacing either the sample or the beam in the x,y directions. Illustrated also are first 114 and second 120 focusing elements, and a 2×2 optical coupler/splitter 112. Lateral resolution is defined by the spot size produced by second focusing element 120.
Conventional OCT and existing commercial products based on the OCT principles outlined above register the z-envelope 144 of the signal (“envelope-only” measurement). The resolution of the depth measurement (imaging) in these tools is comparable to the coherence length Lc.
Enhancement of the z-resolution can be achieved if phase of the signal is measured. An analogy may be made with rangefinders, which have two major classes: those based on time-of-flight of a short radiation pulse and those based on the measurement of phase shifts of long pulses of radiation. Displacement 126 of the reference reflector 116 of a standard OCT setup (FIG. 1) is related to time in this analogy. The OCT measurement resolution-limited by the envelope of the signal is similar to the time-of-flight range measurement, whereas phase-shift measurements in range finding and phase demodulation in OCT have many similarities.
U.S. Pat. No. 5,994,690 discloses an OCT system with signal processing, including phase demodulation, to increase the resolution of depth measurement. The system performs deconvolution of the measured signal by Fourier transform of the measured signal, division by the Fourier transform of the impulse response (instrumental function), and an inverse Fourier transform. The impulse response is obtained by replacing the sample with a reflector and measuring the response of the system. The drawback of the method is the possible effect of spectral variation of scattering by the object. The result of deconvolution also strongly depends on the amount of noise and methods applied for its suppression. In experiments reported in M. D. Kulkarni, J. A. Izatt, Digital signal processing in optical coherence tomography, Coherence domain optical methods in biomedical science and clinical applications, Proc. SPIE; vol. 2981, pp. 2-6, 1997, an increase in resolution by a factor of slightly above 2 was achieved by deconvolution, whereas the number of carrier periods in the envelope was on the order of 10, the latter number being a reasonable expectation of gain in resolution with convolution in the absence of noise. OCT systems with improved signal to noise ratio (SNR) are of interest.
U. Morgner, W. Drexler, F. X. Kartner, X. D. Li, C. Pitris, E. P. Ippen, J. G. Fujimoto, Spectroscopic Optical Coherence Tomography, Optics Letters, 25, 111-113, Jan. 15, 2000, presents a new method for enhanced resolution and extraction of color information using a Mortlet wavelet transform. Along with spectral measurement at every object point, 1 μm longitudinal resolution was achieved.
A typical OCT system known in the art combines 1D coherence scanning and 2D mechanical scanning to produce a 3D profile of the sample. It appears that throughout the prior art, interferometric scanning is performed in one dimension of the sample only, typically designated as the “z” depth dimension. Transverse scanning is performed by mechanical displacement of either the sample, or the beam over the sample surface. In large systems using conventional optics, mechanical beam scanning is typically performed by a deflector such as a galvanometer. In small, fiber optic OCT systems, mechanical rotation of miniature optical elements is used. Such a system is disclosed in U.S. Pat. No. 6,445,939. Single-mode fibers and lenses with diameters substantially the same as the fibers are used in insertable medical devices, such as guidewires. The need for mechanical motion of elements at the tip of the fiber probe adds complexity and cost to the device, and may compromise its range of applications and reliability.
Conventional OCT systems and existing commercial products based on the OCT principle register the envelope of the signal (“envelope-only” measurement). The resolution of the depth measurement (imaging) in these tools is comparable to the coherence length Lc. As noted, for lateral x,y scanning today's “first generation” OCT instruments use mechanical systems to create cross-sectional images of the object. These instruments have several limitations, including:                Bulky probe tip        Slow operation        Limited depth of field        Limited speed of image acquisition        Rigid probe limiting the scope of applications        Large (mm) probe tip diameter, making the use of OCT problematic for many in-vivo imaging tasks        High cost of the OCT system        Low productivity and high cost of procedure        
It would be desirable to provide OCT systems with 3D imaging through optical fibers by 3D coherence gating and scanning, with no mechanical motion at the fiber tip.
It would also be desirable to provide a fiberoptic OCT imaging system applicable to inserted devices such as endoscopes, bronchoscopes, needles, and other similar tools.
It would also be desirable to provide a fiberoptic OCT system with low-cost, removable and disposable fiber probe attachment.
It would also be desirable to provide a new OCT system with improved signal to noise ratio.
Such a system would have many advantages, including:                Three-dimensional, in vivo imaging of tissues and cells in real time at video frame rates        3D coherence scanning with no probe mechanical motion        Focus-invariant imaging for extended depth of field        Improved, micron-level resolution for cellular imaging.        All-fiber, low cross-section, flexible probes—“b 3D Camera Through a Needle”        Low-cost, disposable probes        Affordable, compact, lightweight system        Image acquisition with improved signal-to-noise ratio (SNR) to enable micron and sub-micron imaging resolution        